The present invention relates to an optical pick-up and an objective lens used for an optical disc device, which is capable of recording data to and/or reproducing data from a plurality of types of optical discs having different recording densities and/or having different thicknesses of cover layers.
There are various types of optical discs on which digital information is recorded at various densities, respectively. For example, a DVD (digital versatile disc) has a recording density higher that that of a CD (compact disc) or a CD-R (CD Recordable) and has a cover layer thinner than that of the CD or CD-R.
When a recording/reproducing operation for the DVD having a higher recording density is performed, a smaller beam spot diameter is required on a recording surface of the DVD relative to a beam spot used for the CD having a lower recording density. For this reason, the optical pick-up is configured such that a NA (numerical aperture) is changed to a higher value to obtain a smaller beam spot diameter when the DVD is used and that the NA is changed to a lower value to obtain a larger beam spot diameter when the CD or CD-R is used.
The diameter of the beam spot decreases as a wavelength of the laser beam decreases. Therefore, in general, a laser beam having a wavelength of about 660 nm, which is shorter than a wavelength of about 780 nm for the CD, is used for the DVD. Recently, the optical disc device employing an optical pick-up having a light source unit capable of outputting laser beams having different wavelengths has been used.
A condition of a spherical aberration in an optical system of the optical pick-up changes depending on a thickness of the cover layer of the optical disc being used. Therefore, it is required to correct the spherical aberration caused when the optical disc is changed to another one having different thickness of cover layer while changing the NA in accordance with the type of the optical disc being used.
Japanese Patent Provisional Publication No. HEI 9-306024 (hereafter, referred to as a document 1) discloses an optical system in which magnification of an objective lens is set for each of light beams having different wavelengths so that the light beams are suitably converged onto recording surfaces of plurality of types of optical discs, respectively. In this optical system, the spherical aberration caused by the change of the thickness of the cover layer of the optical disc is cancelled by the spherical aberration caused by the change of a refractive index of the objective lens due to a change of the wavelength.
In the optical system disclosed in the document 1, a converging laser beam is incident on an objective lens when the optical disc having a higher recording density (e.g. DVD) is used. That is, the objective lens supports two types of optical discs.